1. Field of the Invention
The present invention relates generally to electronic mails, and more particularly but not exclusively to computer network security.
2. Description of the Background Art
Electronic mail (“email”) has become a relatively common means of communication among individuals with access to a computer network, such as the Internet. Among its advantages, email is relatively convenient, fast, and cost-effective compared to traditional mail. It is thus no surprise that a lot of businesses and home computer users have some form of email access. Unfortunately, the features that make email popular also lead to its abuse. Specifically, some advertisers have resorted to mass emailings of advertisements over the Internet. These mass emails, which are also referred to as “spam emails” or simply “spam,” are sent to computer users regardless of whether they asked for them or not. Spam includes any unsolicited email, not just advertisements. Spam is not only a nuisance, but is also an economic burden to businesses due to resulting loss in employee productivity.
Some spammers have resorted to malicious means to gather valid email addresses or to send spam. Some of these means, such as directory harvest attacks and bounce-source attacks, not only increase proliferation of spam but also put a heavy load on email systems.